The long term objective of this project is to delineate those factors which determine the transmission of proteins in general and IgG in particular from mother to young. The specific aims of the present application are to: 1) isolate, characterize and localize receptors on the cell membranes in human and rodent placentas and in rodent suckling intestines which selectively and specifically bind proteins as a first step in the transfer of such proteins across these tissue barriers; and 2) determine the kinetics of protein transport in the mammary glands of mice. Cell membranes are to be isolated by differential centrifugation, solubilized and the resulting fragments fractionated by column filtration and by immunochemical binding. The purified fragments are to be characterized chemically. Antisera against the receptor fraction will be prepred for localization on cells. Direct localization of receptors will be determined using labeled proteins and autoradiography. The role of the mammary gland in the selective and specific transfer of IgG and albumin from serum into milk will be determined in mice using I131- labeled purified proteins and total body counting.